In fluidized bed combustion plants with a bed vessel having a grate at the bottom of the bed which includes parallel air distribution tubes and a combustion chamber above the grate and one or more ash chambers with cooled walls below the grate, allowing for the temperature differences arising between the ash chamber and the air distribution tubes, involves constructive problems due to the different thermal expansions of the different structural members. The temperature differences arising in the bed are particularly great during start-up of the plant when combustion gases from a start-up combustion chamber at a temperature of 800.degree.-900.degree. C. are injected through the distribution tubes of the grate for heating the bed material to the auto-ignition temperature of the fuel being used.
In a power plant of the above-described kind, (for example as described in the specification of U.S. Pat. No. 4,268,244) a number of air distribution tubes of circular cross-section are connected individually to an air distribution chamber, which is arranged at right angles to the tubes, by a connecting tube. In an embodiment of the kind shown in U.S. Pat. No. 4,268,244 which is provided with a sufficient crosssection of the air distribution tubes and a suitable distance between these for satisfactory fluidization of the bed and good combustion of fuel, the distance between the air distribution tubes will be small and the gap unsatisfactorily narrow. Slag lumps which form during combustion of the fuel are unable to pass down to the ash chamber(s) below the tubes and the combustion process is thus disturbed. If the slag cannot be cleared during combustion the operation of the plant will eventually have to be interrupted.